Up to 10.3.25 both
SELECT NULL IS NULL = 1;
SELECT (NULL IS NULL) = 1;
worked and returned the same result.
Starting with 10.3.26 only the 2nd variant with extra parenthesis still works, while the 1st variant now throws a syntax error "near '= 1'"
No related entry on the 10.3.26 release notes page refers to such changed parser behavior.